Cancer or tumor cells emerge from the "normal" floura of cells in the body (both human and any other animals known to have tumors). Becoming cancerouse/tumorogenic involves cellular changes. While these changes start off as mutations in the genetic code of the cells, the changes in the behavior of the cell stems from changes in protein/protein expression levels/protein expression.
Structures that are unique to the tumor cell are "tumor antigens". Structures that are not unique to the tumor cell yet might be expressed differently or in access amounts on it are considered "tumor associated antigens". It is wrong to concentrate only on the protein sequence (i.e. also the genomic sequence) as the sole source for changes. Sometimes it is the "coating" of the protein with carbohydrates or lipids that determines its antigenicity. If we take for example the MUCIN from the mammary glands, the protein does not change in the tumor cell, yet antibodies are found against it in patients with breast cancer (and sometimes ovarian cancer too). The reason for it, most probably, is that the protein in the normal cell is not exposed at all due to a dense and thick coat of carbohydrate chains (ie. the protein is very heavily glycosylated). In the tumor cell glycosylation is incomplete thus leading to newly exposed protein sequences that serve (or can serve) as new antigens to the immune system.
Thus any structure that appears to the immune system as new can be considered as a tumor antigen. Another general example of a normal protein appearing and serving as a new antigen for the immune system is that of normal proteins appearing in new context, such as embryonic proteins on mature (or adult) cells. Then the new epitopes would be in the interface between the two structures, an interface that forms new forms and structures for the immune system to see and react to.